Amino acids — practice questions

(a) At pH 1.0 (strongly acidic, pH < pI): The molecule will be fully protonated and have a net positive charge. The carboxylate group accepts a proton. Structure: H₃N⁺–CH(CH₃)–COOH

(b) At pH 6.0 (the pI): The molecule exists as a zwitterion with a net charge of zero. Structure: H₃N⁺–CH(CH₃)–COO⁻

(c) At pH 11.0 (strongly alkaline, pH > pI): The molecule will be deprotonated and have a net negative charge. The ammonium group loses a proton. Structure: H₂N–CH(CH₃)–COO⁻

The two amino acids can join in two different orders: Ala-Gly or Gly-Ala.

1. Dipeptide Ala-Gly (Alanine's –COOH reacts with Glycine's –NH₂): Structure: H₃N⁺–CH(CH₃)–CO–NH–CH₂–COO⁻

2. Dipeptide Gly-Ala (Glycine's –COOH reacts with Alanine's –NH₂): Structure: H₃N⁺–CH₂–CO–NH–CH(CH₃)–COO⁻

Let's circle the peptide bond in the Ala-Gly structure:

H₃N⁺–CH(CH₃)–[CO–NH]–CH₂–COO⁻ (The circled part is the peptide bond)

Note: The structures are shown as zwitterions, which is how they would exist at neutral pH. You may also be asked to draw the un-ionised form, H₂N–CH(R)–CO–NH–CH(R)–COOH.